The delta-sigma modulator is widely used in the fields of audio or communications, and a variety of techniques have so far been proposed for constructing a DA converter or an AD converter using the delta-sigma modulator (see Patent Documents 1 to 3).
FIG. 12 is a block diagram showing an illustrative conventional structure in which the delta-sigma modulator is applied to a D/A converter. In FIG. 12, a digital signal is input to a delta-sigma modulator 121 and thereby modulated into a bi-level (1 bit) output having two levels of +1 and −1. This output is processed by a low-pass filter (LPF) 122 and output as an analog signal.
An output of the delta-sigma modulator is a pulse density modulated input signal having the spectrum containing an input signal spectrum and a spectrum in which the quantization noise is shaped to a frequency range. This output is passed through a low-pass filter (LPF) 122 to take out only the desired frequency range to produce a converted output.
On the other hand, it has also been proposed to provide for a multi-level (multi-bit) output of the delta-sigma modulator, in order to reduce the quantization noise of the delta-sigma modulator to raise its conversion precision or in order to reduce the sampling frequency to achieve high SN and low power consumption (see Patent Documents 1 and 3). With the multi-level output of the delta-sigma modulator, a D/A converter, designed to accommodate the multi-level output, has to be provided as an internal D/A converter for converting the multiple quantization levels into an analog signal.
[Patent Document 1]
    Japanese Patent Kokai Publication No. JP-A-6-224772[Patent Document 2]    Japanese Patent Kokai Publication No. JP-A-6-276099[Patent Document 3]    Japanese Patent Kokai Publication No. JP-P2001-94429A